1. Field of the Invention
The present invention relates to a method for fabricating a circuit device, and in particular, a method for fabricating a thin type circuit device that utilizes a conductive plated layer and conductive layer and is able to achieve multi-layer connection.
2. Description of the Prior Arts
Recently, IC packages have been actively employed in portable devices, and small-sized and high density assembly devices. Conventional IC packages and assembly concepts tend to greatly change. For example, this is described in, for example, Japanese Laid-Open Patent Publication No. 2000-133678. This pertains to a technology regarding a semiconductor apparatus in which a polyimide resin-sheet being a flexible sheet is employed as one example of insulation resin sheets.
FIG. 12 through FIGS. 14A, 14B and 14C show a case where a flexible sheet 50 is employed as an interposer substrate. Also, the views illustrated upside of the respective drawings are plan views, and the views illustrated downside thereof are longitudinally sectional views taken along the lines Axe2x80x94A of the respective drawings.
First, copper foil patterns 51 are prepared to be adhered to each other via an adhesive resin on the flexible sheet 50 illustrated in FIG. 12. These copper foil patterns 51 have different patterns, depending upon cases where a semiconductor element to be assembled is a transistor or an IC. Generally speaking, a bonding pad 51A and an island 51B are formed. Also, an opening 52 is provided to take out an electrode from the rear side of the flexible sheet 50, from which the above-described copper foil pattern 51 is exposed.
Subsequently, the flexible sheet 50 is transferred onto a die bonder, and as shown in FIG. 13, a semiconductor element 53 is assembled or mounted. After that, the flexible sheet 50 is transferred onto a wire bonder, wherein the bonding pads 51A are electrically connected to the pads of the semiconductor elements 53 by thin metal wires 54.
Finally, as shown in FIG. 14A, sealing resin 55 is provided on the surface of the flexible sheet 50, and the surface thereof is completely sealed with the sealing resin 55. Herein, the bonding pads 51A, island 51B, semiconductor elements 53 and thin metal wires 54 are transfer-molded so as to be completely overcoated.
After that, as shown in FIG. 14B, connecting means 56 such as solder and a soldering ball is provided, wherein spherical solder 56 deposited to the bonding pad 51A is formed via the opening 52 by passing through a solder reflow furnace. Further, since semiconductor elements 53 are formed in the form of a matrix on the flexible sheet 50, these are diced to be separated from each other as shown in FIG. 14.
In addition, the sectional view of FIG. 14C shows electrodes 51A and 51D on both sides of the flexible sheet 50 as the electrodes. The flexible sheet 50 is generally supplied from a maker after both sides thereof are patterned.
Since a semiconductor apparatus that employs the above-described flexible sheet 50 does not utilize any publicly known metal frame, the semiconductor apparatus has a problem in that a multi-layer connection structure cannot be achieved while it has an advantage by which a remarkably thin package structure can be brought about, wherein path is carried out with one layer of copper foil pattern 51, which is provided substantially on the surface of the flexible sheet 50.
It is necessary to make the flexible sheet 50 sufficiently thick, for example, approx. 200 xcexcm, in order to retain supporting strength to achieve a multi-layer connection structure. Therefore, there is a problem of retrogression with respect to thinning of the sheet.
Further, in the method for fabricating a circuit device, a flexible sheet 50 is transferred in the above-described fabrication apparatus, for example, a die bonder, wire bonder, a transfer mold apparatus, and a reflow furnace, etc., and the flexible sheet 50 is attached onto a portion called a xe2x80x9cstagexe2x80x9d or a xe2x80x9ctablexe2x80x9d.
However, if the thickness of the insulation resin that becomes the base of the flexible sheet 50 is made thin, for example, 50 xcexcm, the flexible sheet 50 may be warped as shown in FIG. 15 or its transfer performance may be remarkably worsened where the thickness of the copper foil pattern 51 formed on the surface thereof is thin to be 9 through 35 xcexcm. In addition, another problem arises in that the flexible sheet 50 is defectively attached to the above-described stage or table. This is because it is considered that the resin is warped since the insulation resin itself is very thin, and the resin is warped due to a difference in the thermal expansion coefficient between the copper foil pattern 51 and the insulation resin. Particularly, if a hard insulation material in which no glass cloth fibers are used as a core material is warped as shown in FIG. 15, there was a problem in that the insulation material could easily collapse due to compression from above.
Since the portion of the opening 52 is compressed from above when being molded, a force by which the periphery of the bonding pad 51A is warped upward is brought about, the adhesion of the bonding pad 51A is worsened.
Also, the resin material that constitutes a flexible sheet 50 has less flexibility, or if a filler to increase the thermal conductivity is blended, the flexible sheet 50 is made hard. In such a case, where bonding is carried out by a wire bonder, there may be a case where the bonded portion is cracked. Also, when performing transfer molding, there is a case where the portion with which a metal die is brought into contact is cracked. This remarkably occurs if any warping shown in FIG. 15 is provided.
Although the flexible sheet 50 described above is such a type that no electrode is formed on the rear side thereof, there are cases where an electrode 51D is formed on the rear side of the flexible sheet 50 as shown in FIG. 14C. At this time, since the electrode 51D is brought into contact with the above-described fabrication apparatus or is brought into contact with the transfer plane of transfer means between the fabrication apparatuses, another problem occurs in that damage and scratches arise on the rear side of the electrode 51D, wherein the electrode is established with such damage and scratches retained, the electrode 51 itself may be cracked due to application of heat later on.
Also, if an electrode 51D is provided on the rear side of the flexible sheet 50, a problem occurs in that, when carrying out transfer molding, no facial contact with the stage can be secured. In this case, if the flexible sheet 50 is composed of a hard material as described above, the electrode 51D becomes a fulcrum and the periphery of the electrode 51D is compressed downward, wherein the flexible sheet 50 is cracked.
The present inventor proposed use of an insulation resin sheet for which the first thin conductive layer and the second thick conductive layer are adhered by insulation resin.
A method for fabricating a circuit device according to the invention comprises the steps of: preparing an insulation resin sheet having the surface of a conductive layer overcoated with insulation resin; forming through holes in the above-described insulation resin at appointed points on the above-described insulation resin sheet, and selectively exposing the rear side of the above-described conductive layer; forming a conductive plated layer in the above-described through holes and on the surface of the above-described insulation resin; forming a first conductive path layer by etching the above-described conductive plated layer to an appointed pattern; adhering and fixing semiconductor elements on the above-described first conductive path layer with the same electrically insulated therefrom; overcoating the above-described first conductive path layer and the above-described semiconductor elements with a sealing resin layer; forming a second conductive path layer by etching the above-described second conductive layer to an appointed pattern; and forming external electrodes at appointed points of the above-described second conductive path layers, whereby the above-described and other problems can be solved.
Since the flexible sheet is formed to be thick by the conductive layer, the flatness of a sheet-shaped circuit substrate can be maintained even if the insulation resin is thin.
Before the step of overcoating the first conductive path layer and semiconductor elements by a sealing resin layer, the mechanical strength of the first conductive path layer and semiconductor elements is retained by the conductive layer. After that, the mechanical strength is retained by the sealing resin layer. Therefore, it is possible to easily form the second conductive path layer by the conductive layer. As a result, the insulation resin does not need any mechanical strength, wherein it is possible to make the insulation resin thin to the thickness by which electrical insulation can be maintained.
Further, since the lower die mold and planes of a transfer molding apparatus are brought into contact with the entirety of the conductive layer, no local compression is brought about, and it is possible to prevent the insulation resin from being cracked.
The method according to the invention has the following advantages.
First, warping of an insulation resin sheet can be prevented by the conductive layer until a substrate is molded by a sealing resin layer, and transfer performance thereof can be improved.
Second, since a conductive plated layer that forms the first conductive path layer is formed after the through holes, which are formed in the insulation resin, are formed by a carbonic acid gas laser, a multi-layer connection with the second conductive path layer can be simultaneously achieved, and the processes can be remarkably simplified.
Third, the conductive plated layer to form the first conductive path layer can be formed to be thin, and the first conductive path layer can be remarkably finely patterned.
Fourth, since the conductive layer can retain mechanical support of the insulation resin sheet until the sealing resin layer is formed, and the sealing resin layer can subsequently retain mechanical support of the insulation resin sheet after the second conductive path layer is formed, the mechanical strength of the insulation resin is disregarded, wherein a remarkably thin assembly method can be achieved.
Fifth, even when the insulation resin itself is hard or becomes hard by a filler being blended therein, flatness of the insulation resin sheet itself can be increased in the fabrication process since the insulation resin is supported by the conductive layer, and it is possible to prevent cracks from occurring.
Sixth, since the insulation resin sheet has the second conductive layer thickly formed on its rear side, the insulation resin sheet can be utilized as a support substrate for die bonding of chips and for sealing a wire bonder and semiconductor elements. In addition, where the insulation resin material itself is soft, propagation of energy for wire bonding can be improved, and wire bondability can be further improved.